Looking at your mesh, I see a large block meshed with large elements and a small object meshed with small elements, but it doesn't look like the large block has a small object cavity in it. If the large block has no cavity where the small object is, that is wrong. Open the geometry in SpaceClaim and use the Combine tool to subtract the small object from the large block, but keep the small object. Then go to the Workbench tab and click the Share button. By doing that, you won't need any Bonded Contact. The mesh will connect the small object with the face of the cavity in the large block.
In Mechanical, use a Kinematic Mount to connect three corners of the large block to ground. See this discussion on how to do that. /forum/discussion/comment/137006#Comment_137006
Apply a 6 GPa pressure on all six faces.
Create a multi-step analysis.
In Step 1, 6 GPa is applied.
In Step 2, use the INISTATE command to zero the stress in the model. Use the Search button to find information about this command.
In Step 3, change the pressure to 0 GPa. This represents the decompressed state of the rock.